The invention provides polycrystalline silicon.
On the industrial scale, crude silicon is obtained by the reduction of silicon dioxide with carbon in a light arc furnace at temperatures of about 2000° C.
This affords “metallurgical grade” silicon (Simg) having a purity of about 98-99%.
For applications in photovoltaics and in microelectronics, the metallurgical grade silicon has to be purified.
For this purpose, it is reacted, for example, with gaseous hydrogen chloride at 300-350° C. in a fluidized bed reactor to give a silicon-containing gas, for example monosilane or a halosilane, for example trichlorosilane. This is followed by distillation steps in order to purify the silicon-containing gas.
This purified silicon-containing gas then serves as a starting material for the production of high-purity polycrystalline silicon.
The polycrystalline silicon, often also called polysilicon for short, is typically produced by means of the Siemens process. This involves heating thin filament rods of silicon by direct passage of current in a bell-shaped reactor (“Siemens reactor”), with introduction of a reaction gas comprising a silicon-containing component and hydrogen.
The silicon-containing component of the reaction gas is generally monosilane or a halosilane of the general composition SiHnX4−n (n=0, 1, 2, 3; X=Cl, Br, I). It is preferably a chlorosilane, more preferably trichlorosilane. Predominantly SiH4 or SiHCl3 (trichlorosilane, TCS) is used in a mixture with hydrogen.
In the Siemens process, the filament rods are typically inserted perpendicularly into electrodes present at the reactor base, through which they are connected to the power supply. Every two filament rods are coupled via a horizontal bridge (likewise composed of silicon) and form a support body for the silicon deposition. The bridge coupling produces the typical U shape of the carrier bodies, which are also called thin rods.
Polysilicon is deposited on the heated rods and the bridge, as a result of which the rod diameter grows with time (CVD=Chemical Vapor Deposition/gas phase deposition).
After the deposition has ended, these polysilicon rods are typically processed further by means of mechanical processing to give fragments of different size classes, optionally subjected to a wet-chemical purification and finally packed.
The polysilicon can, however, also be processed further in the form of rods or rod pieces. This is especially true for the use of the polysilicon in an FZ process, in which a single crystal is produced from a polysilicon rod.
Polycrystalline silicon (polysilicon for short) serves as a starting material in the production of monocrystalline silicon by means of crucible pulling (Czochralski or CZ process) or by means of zone melting (Floatzone or FZ process).
This monocrystalline silicon is divided into wafers and, after a multitude of mechanical, chemical and chemomechanical processing operations, used in the semiconductor industry for manufacture of electronic components (chips).
More particularly, however, polycrystalline silicon is required for production of mono- or multicrystalline silicon by means of pulling or casting processes, this mono- or multicrystalline silicon serving for production of solar cells for photovoltaics.
Since the quality demands on polysilicon are becoming ever higher, quality control over the entire process chain is indispensible. The material is analyzed, for example, with regard to contaminations with metals or dopants. Contamination in bulk should be distinguished from contamination at the surface of the polysilicon fragments or rod pieces.
It is also customary to convert the polysilicon produced to monocrystalline material for the purposes of quality control. In this case, the monocrystalline material is analyzed. Here too, metal contaminations, which are assessed particularly critically in the customer processes in the semiconductor industry, are of particular significance. The silicon is, however, also analyzed with regard to carbon and dopants such as aluminum, boron, phosphorus and arsenic.
Dopants are analyzed by means of photoluminescence to SEMI MF 1398 on an FZ single crystal produced from the polycrystalline material (SEMI MF 1723). As an alternative, low-temperature FTIR (Fourier Transformer IR spectroscopy) is used (SEMI MF 1630).
FTIR (SEMI MF 1188, SEMI MF 1391) enables the determination of carbon and oxygen concentrations.
The fundamentals of the FZ process are described, for example, in DE-3007377 A.
In the FZ process, a polycrystalline stock rod is gradually melted with the aid of a high-frequency coil, and the molten material is converted to a single crystal by seeding with a monocrystalline seed crystal and subsequent recrystallization. In the course of recrystallization, the diameter of the single crystal forming is first increased in a cone shape (cone formation) until a desired final diameter has been attained (rod formation). In the cone formation phase, the single crystal is also mechanically supported in order to take the load of the thin seed crystal.
In addition, the silicon pulled to a single crystal for analytical purposes is analyzed for its charge carrier lifetime. Different measurement techniques are used here, for example that according to SEMI PV9.
The charge carrier lifetime is crucial for the use of the polycrystalline silicon in the semiconductor and photovoltaics sectors, in order to ensure maximum efficiency of the components.
DE 10 2005 044 328 A1 discloses a polycrystalline silicon material having a lifetime of 2 to 500 μs. For production of the polycrystalline silicon material, the Siemens process is used, and the silane gas used is a trichlorosilane or a monosilane having at least 10 and at most 1000 ppb of boron. The polycrystalline silicon material is used for solar power generation.
According to DE 10 2005 044 328 A1, the purity of silane of semiconductor quality is less than 10 ppb of boron. In addition, the sum of Fe, Cu, Ni, Cr, Zn and Na is 5 ppb or less (ICP method), the donor amount of Al and B is 0.1 ppb or less (photoluminescence), and the lifetime is 1000 μs or more (ASTM F28-91).
EP 0 345 618 B1 discloses a polycrystalline silicon rod with less than 15 ppta of boron and less than 20 ppta of phosphorus. Such a rod can be used to produce, by means of floating zone processes, monocrystalline silicon having a specific resistivity of at least 10 000 ohmcm and a lifetime of at least 10 000 μs.
The polycrystalline silicon rod is produced by means of the Siemens process. The polycrystalline silicon rod contains less than 5 ppta of aluminum and less than 0.1 ppma of carbon. The filament rod contains less than 0.2 ppba of boron and less than 0.2 ppba of phosphorus.
In the Siemens process in EP 0 345 618 B1, preference is given to using monosilane. The reason given for this is that boron and phosphorus can easily be removed from monosilane. In contrast, trichlorosilane does not allow effective distillation because the adsorbents used contaminate the trichlorosilane with aluminum.
This problem gave rise to the objective of the invention.